Phospho-selective mechanisms of arrestin conformations and functions revealed by unnatural amino acid incorporation and (19)F-NMR.
ABSTRACT: Specific arrestin conformations are coupled to distinct downstream effectors, which underlie the functions of many G-protein-coupled receptors (GPCRs). Here, using unnatural amino acid incorporation and fluorine-19 nuclear magnetic resonance ((19)F-NMR) spectroscopy, we demonstrate that distinct receptor phospho-barcodes are translated to specific ?-arrestin-1 conformations and direct selective signalling. With its phosphate-binding concave surface, ?-arrestin-1 'reads' the message in the receptor phospho-C-tails and distinct phospho-interaction patterns are revealed by (19)F-NMR. Whereas all functional phosphopeptides interact with a common phosphate binding site and induce the movements of finger and middle loops, different phospho-interaction patterns induce distinct structural states of ?-arrestin-1 that are coupled to distinct arrestin functions. Only clathrin recognizes and stabilizes GRK2-specific ?-arrestin-1 conformations. The identified receptor-phospho-selective mechanism for arrestin conformation and the spacing of the multiple phosphate-binding sites in the arrestin enable arrestin to recognize plethora phosphorylation states of numerous GPCRs, contributing to the functional diversity of receptors.
Project description:G-protein-coupled receptor (GPCR) ligands impart differing degrees of signaling in the G-protein and arrestin pathways, in phenomena called "biased signaling". However, the mechanism underlying the biased signaling of GPCRs is still unclear, although crystal structures of GPCRs bound to the G protein or arrestin are available. In this study, we observed the NMR signals from methionine residues of the μ-opioid receptor (μOR) in the balanced- and biased-ligand-bound states. We found that the intracellular cavity of μOR exists in an equilibrium between closed and multiple open conformations with coupled conformational changes on the transmembrane helices 3, 5, 6, and 7, and that the population of each open conformation determines the G-protein- and arrestin-mediated signaling levels in each ligand-bound state. These findings provide insight into the biased signaling of GPCRs and will be helpful for development of analgesics that stimulate μOR with reduced tolerance and dependence.
Project description:Arrestins regulate the signaling of ligand-activated, phosphorylated G-protein-coupled receptors (GPCRs). Different patterns of receptor phosphorylation (phosphorylation barcode) can modulate arrestin conformations, resulting in distinct functional outcomes (for example, desensitization, internalization, and downstream signaling). However, the mechanism of arrestin activation and how distinct receptor phosphorylation patterns could induce different conformational changes on arrestin are not fully understood. We analyzed how each arrestin amino acid contributes to its different conformational states. We identified a conserved structural motif that restricts the mobility of the arrestin finger loop in the inactive state and appears to be regulated by receptor phosphorylation. Distal and proximal receptor phosphorylation sites appear to selectively engage with distinct arrestin structural motifs (that is, micro-locks) to induce different arrestin conformations. These observations suggest a model in which different phosphorylation patterns of the GPCR C terminus can combinatorially modulate the conformation of the finger loop and other phosphorylation-sensitive structural elements to drive distinct arrestin conformation and functional outcomes.
Project description:G-protein-coupled receptors (GPCRs) can bias signaling through distinct biochemical pathways that originate from G-protein/receptor and ?-arrestin/receptor complexes. Receptor conformations supporting ?-arrestin engagement depend on multiple receptor determinants. Using ghrelin receptor GHR1a, we demonstrate by bioluminescence resonance energy transfer and fluorescence microscopy a critical role for its second intracellular loop 2 (ICL2) domain in stabilizing ?-arrestin/GHSR1a core interactions and determining receptor trafficking fate. We validate our findings in ICL2 gain- and loss-of-function experiments assessing ?-arrestin and ubiquitin-dependent internalization of the CC chemokine receptor, CCR1. Like all CC and CXC subfamily chemokine receptors, CCR1 lacks a critical proline residue found in the ICL2 consensus domain of rhodopsin-family GPCRs. Our study indicates that ICL2, C-tail determinants, and the orthosteric binding pocket that regulates ?-arrestin/receptor complex stability are sufficient to encode a broad repertoire of the trafficking fates observed for rhodopsin-family GPCRs, suggesting they provide the essential elements for regulating a large fraction of ?-arrestin signaling bias.
Project description:Beta-arrestins critically regulate G protein-coupled receptors (GPCRs), also known as seven-transmembrane receptors (7TMRs), both by inhibiting classical G protein signaling and by initiating distinct beta-arrestin-mediated signaling. The recent discovery of beta-arrestin-biased ligands and receptor mutants has allowed characterization of these independent "G protein-mediated" and "beta-arrestin-mediated" signaling mechanisms of 7TMRs. However, the molecular mechanisms underlying the dual functions of beta-arrestins remain unclear. Here, using an intramolecular BRET (bioluminescence resonance energy transfer)-based biosensor of beta-arrestin 2 and a combination of biased ligands and/or biased mutants of three different 7TMRs, we provide evidence that beta-arrestin can adopt multiple "active" conformations. Surprisingly, phosphorylation-deficient mutants of the receptors are also capable of directing similar conformational changes in beta-arrestin as is the wild-type receptor. This indicates that distinct receptor conformations induced and/or stabilized by different ligands can promote distinct and functionally specific conformations in beta-arrestin even in the absence of receptor phosphorylation. Our data thus highlight another interesting aspect of 7TMR signaling--i.e., functionally specific receptor conformations can be translated to downstream effectors such as beta-arrestins, thereby governing their functional specificity.
Project description:Phosphorylation of G protein-coupled receptors (GPCRs, which are also known as seven-transmembrane spanning receptors) by GPCR kinases (GRKs) plays essential roles in the regulation of receptor function by promoting interactions of the receptors with ?-arrestins. These multifunctional adaptor proteins desensitize GPCRs, by reducing receptor coupling to G proteins and facilitating receptor internalization, and mediate GPCR signaling through ?-arrestin-specific pathways. Detailed mapping of the phosphorylation sites on GPCRs targeted by individual GRKs and an understanding of how these sites regulate the specific functional consequences of ?-arrestin engagement may aid in the discovery of therapeutic agents targeting individual ?-arrestin functions. The ?(2)-adrenergic receptor (?(2)AR) has many serine and threonine residues in the carboxyl-terminal tail and the intracellular loops, which are potential sites of phosphorylation. We monitored the phosphorylation of the ?(2)AR at specific sites upon stimulation with an agonist that promotes signaling by both G protein-mediated and ?-arrestin-mediated pathways or with a biased ligand that promotes signaling only through ?-arrestin-mediated events in the presence of the full complement of GRKs or when either GRK2 or GRK6 was depleted. We correlated the specific and distinct patterns of receptor phosphorylation by individual GRKs with the functions of ?-arrestins and propose that the distinct phosphorylation patterns established by different GRKs establish a "barcode" that imparts distinct conformations to the recruited ?-arrestin, thus regulating its functional activities.
Project description:Extracellular ligand binding to G protein-coupled receptors (GPCRs) modulates G protein and ?-arrestin signaling by changing the conformational states of the cytoplasmic region of the receptor. Using site-specific (19)F-NMR (fluorine-19 nuclear magnetic resonance) labels in the ?(2)-adrenergic receptor (?(2)AR) in complexes with various ligands, we observed that the cytoplasmic ends of helices VI and VII adopt two major conformational states. Changes in the NMR signals reveal that agonist binding primarily shifts the equilibrium toward the G protein-specific active state of helix VI. In contrast, ?-arrestin-biased ligands predominantly impact the conformational states of helix VII. The selective effects of different ligands on the conformational equilibria involving helices VI and VII provide insights into the long-range structural plasticity of ?(2)AR in partial and biased agonist signaling.
Project description:Arrestins desensitize G protein-coupled receptors (GPCRs) and act as mediators of signalling. Here we investigated the interactions of arrestin-1 with two functionally distinct forms of the dim-light photoreceptor rhodopsin. Using unbiased scanning mutagenesis we probed the individual contribution of each arrestin residue to the interaction with the phosphorylated apo-receptor (Ops-P) and the agonist-bound form (Meta II-P). Disruption of the polar core or displacement of the C-tail strengthened binding to both receptor forms. In contrast, mutations of phosphate-binding residues (phosphosensors) suggest the phosphorylated receptor C-terminus binds arrestin differently for Meta II-P and Ops-P. Likewise, mutations within the inter-domain interface, variations in the receptor-binding loops and the C-edge of arrestin reveal different binding modes. In summary, our results indicate that arrestin-1 binding to Meta II-P and Ops-P is similarly dependent on arrestin activation, although the complexes formed with these two receptor forms are structurally distinct.
Project description:Despite intense interest in discovering drugs that cause G-protein-coupled receptors (GPCRs) to selectively stimulate or block arrestin signalling, the structural mechanism of receptor-mediated arrestin activation remains unclear1,2. Here we reveal this mechanism through extensive atomic-level simulations of arrestin. We find that the receptor's transmembrane core and cytoplasmic tail-which bind distinct surfaces on arrestin-can each independently stimulate arrestin activation. We confirm this unanticipated role of the receptor core, and the allosteric coupling between these distant surfaces of arrestin, using site-directed fluorescence spectroscopy. The effect of the receptor core on arrestin conformation is mediated primarily by interactions of the intracellular loops of the receptor with the arrestin body, rather than the marked finger-loop rearrangement that is observed upon receptor binding. In the absence of a receptor, arrestin frequently adopts active conformations when its own C-terminal tail is disengaged, which may explain why certain arrestins remain active long after receptor dissociation. Our results, which suggest that diverse receptor binding modes can activate arrestin, provide a structural foundation for the design of functionally selective ('biased') GPCR-targeted ligands with desired effects on arrestin signalling.
Project description:G protein-coupled receptors (GPCRs) mediate diverse signaling in part through interaction with arrestins, whose binding promotes receptor internalization and signaling through G protein-independent pathways. High-affinity arrestin binding requires receptor phosphorylation, often at the receptor's C-terminal tail. Here, we report an X-ray free electron laser (XFEL) crystal structure of the rhodopsin-arrestin complex, in which the phosphorylated C terminus of rhodopsin forms an extended intermolecular ? sheet with the N-terminal ? strands of arrestin. Phosphorylation was detected at rhodopsin C-terminal tail residues T336 and S338. These two phospho-residues, together with E341, form an extensive network of electrostatic interactions with three positively charged pockets in arrestin in a mode that resembles binding of the phosphorylated vasopressin-2 receptor tail to ?-arrestin-1. Based on these observations, we derived and validated a set of phosphorylation codes that serve as a common mechanism for phosphorylation-dependent recruitment of arrestins by GPCRs.
Project description:(?-)Arrestins are important regulators of G-protein-coupled receptors (GPCRs). They bind to active, phosphorylated GPCRs and thereby shut off 'classical' signalling to G proteins, trigger internalization of GPCRs via interaction with the clathrin machinery and mediate signalling via 'non-classical' pathways. In addition to two visual arrestins that bind to rod and cone photoreceptors (termed arrestin1 and arrestin4), there are only two (non-visual) ?-arrestin proteins (?-arrestin1 and ?-arrestin2, also termed arrestin2 and arrestin3), which regulate hundreds of different (non-visual) GPCRs. Binding of these proteins to GPCRs usually requires the active form of the receptors plus their phosphorylation by G-protein-coupled receptor kinases (GRKs). The binding of receptors or their carboxy terminus as well as certain truncations induce active conformations of (?-)arrestins that have recently been solved by X-ray crystallography. Here we investigate both the interaction of ?-arrestin with GPCRs, and the ?-arrestin conformational changes in real time and in living human cells, using a series of fluorescence resonance energy transfer (FRET)-based ?-arrestin2 biosensors. We observe receptor-specific patterns of conformational changes in ?-arrestin2 that occur rapidly after the receptor-?-arrestin2 interaction. After agonist removal, these changes persist for longer than the direct receptor interaction. Our data indicate a rapid, receptor-type-specific, two-step binding and activation process between GPCRs and ?-arrestins. They further indicate that ?-arrestins remain active after dissociation from receptors, allowing them to remain at the cell surface and presumably signal independently. Thus, GPCRs trigger a rapid, receptor-specific activation/deactivation cycle of ?-arrestins, which permits their active signalling.